Don't think much of BVB's 4-3-1-2.
It's not a system I like generally and they don't have they players to play it just now.
To play it in a non-counter-attacking fashion, you'd better have some technically gifted midfielders who are going to allow you to play through the middle of the pitch.
Without Hummels and Gundogan, BVB couldn't even start to build attacks.
Good call from Leverkusen to press high up the pitch to deny BVB's technically mediocre time on the ball that might have allowed them to start building things up as they wanted to.